A magnetic resonance imaging (MRI) apparatus magnetically excites nuclear spins of an object placed in a static magnetic field with an RF signal at Larmor frequency to reconstruct an image using a magnetic resonance signal resulting from the excitation.
For this type of MRI apparatus, it is important to reduce various impacts such as eddy currents which reduce image quality of images.
Static magnetic field strength of the MRI apparatus might fluctuate due to various factors such as execution of a pulse sequence or variation with time. On the other hand, the static magnetic field strength is proportionally related to resonant frequency. Consequently, if the static magnetic field strength fluctuates, when any gradient magnetic field is applied, a frequency shift corresponding to an amount of the gradient magnetic field might occur, causing fluctuations of center frequency. An amount of fluctuation varies with a state of adjustment of the gradient coil and a phase of atomic nuclei excited during imaging is modulated by this phenomenon, resulting in losses of image quality in the form of a reduced SNR and sensitivity irregularities. Therefore, if frequency shifts occur, preferably amounts of the frequency shifts are measured quantitatively to reduce impacts of the frequency shifts on the image quality.